A conventional fluid control circuit for selectively engaging service brakes employed on a construction vehicle or the like, normally includes a directional control valve interconnected between an engine-driven pump and the brakes. The brakes may be of the normally engaged type whereby the spring-biased brakes are released upon communication of pressurized hydraulic fluid to actuating chambers thereof. In addition, the control circuit may include separate parking and retarding valves for controlling engagement of the brakes.
The latter valves are normally connected directly to the inlet to the control valve and thus must utilize relatively high volume lines for selectively venting hydraulic fluid from the actuating chambers of the brakes. Alternatively, relatively complex mechanical linkages may be interconnected between an operator control lever and a spool of the control valve to move the spool to an open position venting hydraulic fluid from the actuating chambers of the brakes. Conventional systems of the above type are thus somewhat complex and do not provide for remote location of the brake and retarder valves, adjacent to the operator in the cab of the vehicle.
In addition, conventional braking systems of the above type do not interassociate the parking and retarding valves with each other whereby they may be placed in close proximity for ready access by the operator. The inability to interassociate the parking and retarder valves further complicates the overall control circuit, as will be appreciated by those skilled in the arts relating hereto. In addition, retarder valves are normally of the reciprocal spool type, generally held in set positions by detent mechanisms or the like. Thus, it has proven difficult with these types of valves to infinitely vary the retarding capacity of the brakes of the vehicle.